1. Field of the Invention
This invention concerns a current sensing circuit using FET (Field Effect Transistor). More particularly, this invention concerns a current sensing circuit which is used as an over current sensing circuit in a power circuit.
2. Description of the Prior Art
It is often desirable to sense a load current in order to either control that current or to prevent it from exceeding certain bounds. Conventionally, a current sensing circuit which includes a resistor connected in series with the load is often used. In this circuit, the load current is sensed by sensing the voltage drop at the resistor. However, by inserting a resistor in a high current path, the power consumption is increased.
Thus, to reduce the power consumption, a current sensing circuit using a main FET and a sense FET is disclosed in U.S. Pat. No. 4,553,084.
FIG. 1 shows a circuit diagram of the current sensing circuit disclosed in the United States Patent. In the current sensing circuit, a main FET 1, having an input drive circuit 3 is connected in series with a load 4 between a first power source terminal 6 and a second power source terminal 7. A sense FET 2 is connected in parallel with the main FET 1. This connection allows a second current path for part of the load current which flows through the load 4. A comparator 8 compares the voltage drop at a resistor 5 with a reference voltage supplied to a terminal 9, and produces an output signal from an output terminal 10 in accordance with the comparison result.
In this circuit, a small resistance is required for the sense resistor 5 to keep good linearity of the current which flows through resistor 5 with respect to the load current. Namely, the accuracy of the resistor 5 determines the accuracy of the sensing. However, it is not so easy to provide a resistor with an accurate but small resistance value.